Small Computer System Interface, or SCSI (pronounced scuzzy), is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it can connect a wide range of other devices, including scanners and CD drives.
Sata is the type of connection that the hard drive has to the computer. Its sort of like a boat. your hard drive is the Boat (say a Speedboat). The computers motherboard is the dock. You tie the boat to the dock with a simple nylon rope. The dock (your motherboard) doesnt see the boat, but it does see that your using a Nylon Rope. The operating system however, is the security camera. The security camera can see that you have a speedboat, hooked up to the dock.
I hope that makes sense... It sounded better in person...